I can't stop laughing!
Last week or two I've been monitoring my site
www.sochoose.com at yahoo.com (exact same SE every time) for search term "employee assistance programme".
In the mornings (UK) it appears between 127 and 129, in the afternoons between 27 and 29. I've even located the exact time 12:10 PM, that this happens.
I get virtually zero traffic from Yahoo anyway so not especially bothered at the moment, though I could imagine tearing my hair out if we ranged from, say 5-105.
The answer then is simple! Yahoo changes between AM and PM results GMT.
pne